The prior art is replete with a wide variety of both inorganic and organic cement accelerators. In a co-pending application, Ser. No. 07/131,878, now U.S. Pat. No. 4,883,125 entitled "CEMENTING OIL AND GAS WELLS USING CONVERTED DRILLING FLUID" of which the present co-inventors are co-inventors and having the same assignee, there was discussed the completion of oil and gas wells by converting of a drilling fluid, or "mud", to a cement, and several patents were cited therein which disclose compositions for accomplishing this.
In that application it was noted that efforts to convert drilling fluid containing cementitious materials have posed problems such as increased viscosity due to flocculation as cementitious material is added to the drilling mud, and subsequently pumped into the wellbore. Therein was cited U.S. Pat. No. 3,499,491, which describes difficulties with gelling characteristics and, particularly, temperature sensitivity.
In both primary and remedial applications, it is also sometimes desirable to set retarded cement formulations more rapidly. Typically cement is retarded for pumping into annular voids or perforations where it is desirable to have a controlled viscosity and set time to permit safe cement slurry placement, but also desirable to have the slurry set rapidly after placement. For example, to allow ample time for placement of cement formulations before set, retarders are frequently added to prevent the development of high viscosities or premature setting of cement slurry during the pumping and squeezing into the annular voids or perforations. Yet, these retarders can also delay the setting of the cement for long periods of time after placement even under in situ conditions. Excessive set times contribute to high operational costs due to rig time standby. Thus, it is desirable that a compatible accelerator be employed in some fashion to greatly reduce the set time for the cement.
The same is true when setting cement in the form of plugs in the wellbore. This may be for plugging the well; formation pack off, or seal off between productive formations or otherwise; and/or for wellbore deviation. Frequently, the cement plug intermingles or becomes contaminated with the drilling fluid. The drilling fluid is an effective cement set retarder and prevents or delays the set of the cement plug. In any event, it is sometimes desirable that an over wash be provided to accelerate the set of the cement strength development without adversely affecting pumping time.